Hispaniola is one of the Greater Antilles in the
Northern Caribbean Sea. The island, which includes both Haiti and
the Dominican Republic, is home to 22 million people.
The Caribbean Plate moves east at about two centimeters per year with
respect to the North American Plate. In this region the Caribbean Plate is
broken into at least four microplates that accommodate the
transition between the major plates. Subduction of North American lithosphere
beneath the Caribbean Plate changes from frontal subduction
at the Lesser Antilles Trench to oblique subduction at the Puerto Rico Trench
to oblique collision at the Hispaniola Trench
and then to strike-slip motion between Cuba and Jamaica.
In order to examine how North American plate motion drives deformation across
the 280-kilometer-wide Hispaniola Microplate
we will hold the Caribbean plate fixed. In general,
the Atlantic Ocean crust is only six kilometers thick.
But at the Bahamas carbonate platform, limestone-reef deposits up to six
kilometers thick have doubled the crustal thickness with more
buoyant material making it more difficult to subduct.
This sector of the subduction zone is called an oblique
collision zone because as the Bahamas platform is subducted with the
North American Plate, the high friction shoves and deforms the
overriding Northern Hispaniola microplate
like a small scale continent-continent collision.
Subduction of this thicker crust also results in extension
on the boundary between the Hispaniola and Puerto Rico-Virgin Islands
microplates where the magnitude 7.5 Mona Passage
earthquake occurred in 1918. Oblique collision at the Hispaniola
Trench results in strain partitioning. Shear strain is accommodated by the
Septentrianol and Enriquillo trike slip fault zones.
Compressional strain is taken up by subduction on the mega
thrust plate boundary at the Hispaniola Trench and by thrust
faulting at the Muertos Trough where the Caribbean Plate pushes back.
The rate of compression is about 10 millimeters per year.
In subduction zones we expect to find shallow earthquakes near the trench and
bands of deeper and deeper earthquakes as the subducting plate dives deeper
into the mantle. In Hispaniola, this expected pattern is
complicated by the thrust faulting at the Muertos Trough.
A north-south cross section shows the Hispaniola Microplate
over-thrusting Caribbean lithosphere at the Muertos Trough,
presenting an earthquake and tsunami hazard to Southern Hispaniola.
In 1984, a magnitude 6.9 earthquake occurred on the Muertos
thrust fault but did not reach the surface.
Subduction of North American oceanic lithosphere beneath Northern Hispaniola
is at a shallow angle which then turns nearly vertical to 200 kilometers
depth where the deepest earthquakes occur. How the two oceanic plates
interact at depth is unknown. The latest major earthquake on the
subduction megathrust occurred on august 4th 1946.
That magnitude 7.9 earthquake initiated at 20 kilometers depth
and ruptured up the plate boundary to the Hispaniola Trench
with megathrust displacement reaching two meters.
This may be the largest earthquake on the Caribbean—North American Plate
boundary in the 500-year written historical record.
The resulting tsunami arrived on the northeast coast of the
Dominican Republic with wave heights of two to five meters
and was measured by tide gauges as far north as New Jersey.
Eyewitnesses, emergency managers and relief workers,
reported that the tsunami ran almost a kilometer inland
and swept the town of Mantansitas and several villages into the ocean
resulting in 1790 fatalities. But the Trujillo administration insisted
on reporting an official death toll of 100. left lateral shear strain across
Hispaniola occurs at a rate of 17 millimeters per year.
Recent GPS observations document left lateral strike slip rates of
9 millimeters per year on the Septentrionol Fault zone and 11
millimeters per year on the Enriquillo Fault zone. With only a
few decades of seismometer recordings we must rely on written historical
records of earthquakes on these strike slip faults over the past few centuries
a series of earthquakes on the Enriquillo Fault zone from 1751 to 1770
was followed by the devastating magnitude 7 earthquake on january 12
2010. This limited history suggests a recurrence interval of 240 years for
major earthquakes or a series of earthquakes on the
Enriquillo Fault zone. The earthquakes of 1842 and 1887
most likely occurred on the western Septentrional fault zone.
However, these locations are approximate and we don't know how far east they
ruptured leaving the earthquake history along
this section uncertain. Some researchers have concluded that the
entire zone has a recurrence interval of about 300 years
with a characteristic earthquake of magnitude 7.5 to 8
and a strike slip offset of two to four meters.
However, paleoseismic analysis from deposits and trenches across the
Septentrional Fault zone in central Hispaniola have been interpreted to indicate that
the most recent earthquake in that area occurred between 1040 and 1230 AD,
indicating a recurrence interval of perhaps a thousand years.
It could be that 10 meters of strain has accumulated since the last earthquake
and the next earthquake could exceed magnitude 8.
Despite these uncertainties it's clear that the Septentrionol fault zone will
have earthquakes equal to or larger than the 2010 Haiti earthquake.
This simplified map of population density illustrates how the
Septentrionol and Enriquillo fault zones pass near or through
major population centers in Haiti and
the Dominican Republic. In addition, mega thrust earthquakes
along the Hispaniola thrust fault and on the Muertos Trough pose high
earthquake hazard and tsunami hazards to this and the
surrounding areas of the Caribbean.